The transactivator of the transcription gene product (tat) of human immunodeficiency virus-1 (HIV-1) may modulate cellular gene expression. Infection of H9 cells with HIV-1 is demonstrated to produce changes in the level of expression of some serum-inducible genes, although oncogene expression appears to be unaffected. Currently, uncharacterized genes (cDNA clones) whose expression may be repressed have been isolated by differential hybridization of infected vs. uninfected H9 cell cDNA libraries. A construct in which functional tat can be expressed under control of the heat shock promoter is being used to construct permanent cell lines.